Date of Award

2023

Degree Type

Dissertation

Degree Name

Doctor of Philosophy in Mechanical Engineering and Applied Mechanics

Department

Mechanical, Industrial and Systems Engineering

First Advisor

Arun Shukla

Abstract

The ever-advancing stage of modern warfare poses a threat to civilian and defense structures, necessitating the research and understanding of new, blast-resistant structural materials. In response to this need, composite and composite sandwich structures have been developed and are finding important applications in the naval and aerospace industry. Naval and marine communities worldwide are increasingly interested in using composite materials for construction, expanding their use beyond small surface boats to include submarines, deckhouses, and unmanned underwater vehicles. Composite materials offer several advantages over metallic materials in marine applications, including higher strength-to-weight ratios, reduced maintenance, better corrosion resistance, and improved acoustic attenuation. As a result, composites are already used in military applications such as armored Army vehicles, Navy sonar domes, and hull sheathings. However, the lack of complete understanding and simple design rules for the use of composite and sandwich structures, especially under extreme loading conditions, remains a significant hindrance to their widespread application. The focus of this work involved utilizing advanced experimental methods and equipment to characterize, evaluate, and develop understanding towards use of composite materials and structures in blast-mitigation. The objective was to generate valuable and unique experimental data that would contribute to the development of blast mitigation strategies and advanced computational and numerical models. The advancements made in these areas were expected to directly enhance the structural designs for naval structures. This work aimed to expand on existing knowledge and build upon previous blast mitigation research conducted at URI. One aspect of the research involved designing and creating composite panels using materials like CFRP and closed cell polymeric foams. This included the fabrication of sandwich structures utilizing various foams such as polymer, chemical, syntactic, and functionally graded foams. Another area of focus was conducting high strain rate tests. These tests involved subjecting foams and polymeric coatings to high strain rates, as well as characterizing the behavior of composite materials, specifically the face-sheets, under such conditions. Furthermore, shock testing of composite panels was carried out. This entailed using instrumented shock tubes to simulate shock loading and conducting UNDEX (underwater explosion) tests with air-backed conditions on flat composite panels.

Chapter 1 provides a thorough review of the state of the art around blast mitigation available at beginning of this work. This review focuses on achieving effective blast mitigation using polymeric composites. The study highlights the differences in loading mechanisms between AIREX and UNDEX and identifies the ways in which polymers can mitigate these two loading modes. This understanding facilitates the development of a tailored design approach for blast mitigation. The review also discusses recent research on blast mitigation in composite sandwich architecture, with emphasis on the impact of various components on mitigation performance. Additionally, the use of additively manufactured applications, auxetics, and foam metamaterial constructions for blast mitigation purposes is explored.

Chapter 2, Naval structures get subjected to highly dynamic loadings such as blast and shock loadings, severe environmental conditions, including exposure to seawater, moisture, extreme temperatures such as in the Arctic regions, and fluid-structure interactions. When commissioned under deep sea environments enclosed naval structures are also vulnerable to structural dynamic instability due to hydrostatic loading which leads to implosion and catastrophic failure. This review examines experimental, analytical, and numerical efforts focused on understanding the damage and deformation mechanisms associated with blast loading in naval sandwich structures and the mechanics of implosion in underwater naval structures. The study also explores state-of-the-art methods and techniques developed to mitigate the catastrophic effects of blasts and implosion on naval structures.

Chapter 3, Closed cell polymeric foams are commonly used as core materials in naval sandwich structures due to their lightweight and high energy absorption capabilities. However, current understanding of their material response is limited to uniaxial or multiaxial air loadings at different strain rates. This chapter investigates the hydrostatic elastic response and yield behavior of PVC foams with varying mass densities under high strain rate hydrostatic loading conditions. A novel underwater high strain rate loading facility is utilized, along with 3-D Digital Image Correlation (DIC) and dynamic pressure sensors, to measure deformation behavior and material properties. The study reveals an increase in bulk modulus and yield strength of foams under high strain rate loading, with sensitivity to foam mass density.

Chapter 4, Polymeric foams are widely used in sandwich structures to withstand blast loadings, serving as energy dissipators and force attenuators. This study explores the mechanics involved when an air shock interacts with closed-cell polymeric foam. The authors provide a comprehensive analysis of stress wave propagation and its evolution across the foam specimen by employing ultra-high-speed imaging and digital image correlation (DIC). Measurement of elastic, plastic, and shock wave velocities inside the impacted foam was conducted. A methodology to predict elastic precursor wave velocity is proposed, that considers the foam material's behavior at higher strain rates. The predicted values were found to be in excellent agreement with the experimentally obtained values. Furthermore, the authors investigate the force transmission through foams as a function of bulk foam density under unconfined conditions. The results reveal that under shock loading intensities where foam specimens remained in their elastic regime, a transmitted force amplification was observed relative to the input load. However, when subjected to higher-intensity shock loading, substantial foam deformation occurs during the initial propagation of the shock wave. In these cases, the transmitted force measured was found to rely on the plastic and shock waves generated within the material. Additionally, as the loading persists, the force transmissibility ratio decreases beyond the initial stress wave propagation phase, thus demonstrating the material's suitability for blast mitigation applications.

Chapter 5, The effect of rubber additives on the shock response and failure mechanism of composite panels subjected to air shock loading is investigated in this chapter. Material fabrication, characterization, and analysis of composite panels' response and damage are conducted under various shock loadings. The study quantifies the effect of rubber modification on structural damping and energy dissipation, indicating improved performance under below-failure shock loadings and limited crack growth under high-intensity shock loadings.

Chapter 6, This chapter presents a novel approach to analyze the effects of near-field underwater blast loading on composite marine structures. The study focuses on the propagation of spherical blast waves and subsequent secondary bubble collapse pulses. An analytical framework is developed to calculate blast energies and structural energies accurately during this complex loading event. Experiments are conducted to characterize the blast loading and the interaction between the blast and the structure. The results highlight the significant damaging effects of bubble collapse during near-field underwater blast loadings and demonstrate the effectiveness of low-density closed-cell foam cladding in mitigating blast loading on composite structures.

Chapter 7 provides a short summery along with future scope of the work discussed in the thesis.

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